In recent years researchers have recognized that emphasizing between- family differences in children's developmental outcomes has created a gap in knowledge about important within-family variation. The goal of the present study is to describe the distribution of mental health symptomatology within families with children with behavior problems. Data for this project are from the longitudinal study of children at risk for the development of disruptive behavior disorders (88-M-0217). The primary research question concerns the level of concordance between siblings and comorbidity within children for behavioral symptomatology. Behavioral symptomatology includes both externalizing (aggressive, impulsive, overactive) and internalizing (depressed, anxious, compulsive) problems. Resemblance between siblings in personality and psychopathology is assumed to be attributable to primarily genetic sources (including gene-environment correlations). Therefore the presence or absence of parental symptomatology will be crossed with the concordance or discordance of siblings for behavior problems. It is hypothesized that families with at least one maladjusted parent will be more likely to have siblings concordant for behavioral problems. Among families without maladjusted parents, the presence of significant oppositional behavior in one child has been shown in some research to be protective for other children in the same family. Evidence for this second pattern will also be examined. Data have been collected for the present investigation and are currently being prepared for analysis.